Solar energy is one of the most important energy sources that have become available in recent years. Considerable research and development have been conducted in silicon-based solar cell semiconductor materials and solar cell structural technologies. As a result, advanced semiconductor solar cells have been applied to a number of commercial and consumer-oriented applications. For example, solar technology has been applied to satellites, space, mobile communications, and so forth.
Energy conversion from solar energy or photons to electrical energy is a critical issue in the generation of solar energy. For example, in satellite and/or other space related applications, the size, mass, and cost of a satellite power system are directly related to the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of solar power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power generation devices for the on-board power systems, become increasingly more important.
The efficiency of energy conversion, which converts solar energy (or photons) to electrical energy, depends on various factors such as solar cell structures, semiconductor materials, et cetera. In other words, the energy conversion for each solar cell is dependent on the effective utilization of the available sunlight across the solar spectrum. As such, the characteristic of sunlight absorption in semiconductor material, also known as photovoltaic properties, is critical to determine the efficiency of energy conversion.
Conventional solar cells typically use compound materials such as indium gallium phosphide (InGaP), gallium arsenic (GaAs), germanium (Ge) and so forth, to increase coverage of the absorption spectrum from UV to 890 nm. For instance, addition of a germanium (Ge) junction to the cell structure extends the absorption range (i.e. to 1800 nm). Thus, the selection of semiconductor compound materials can enhance the performance of the solar cell.
Physical or structural design of solar cells can also enhance the performance and conversion efficiency of solar cells. Solar cells have been typically designed in multijunction structures to increase the coverage of the solar spectrum. Solar cells are normally fabricated by forming a homojunction between an n-type layer and a p-type layer. The thin, topmost layer of the junction on the sunward side of the device is referred to as the emitter. The relatively thick bottom layer is referred to as the base. However, a problem associated with the conventional multijunction solar cell structure is low performance relating to the homojunction middle solar cells in the multijunction solar cell structures. The performance of a homojunction solar cell is typically limited by the material quality of the emitter, which is low in homojunction. Low material quality usually includes poor surface passivation, lattice miss-match, and/or narrow band gap.
Thus, a mechanism is needed to enhance the performance of multijunction solar cell structures.